Fermentation may be broadly defined as the controlled cultivation of microorganisms for the transformation of an organic compound into a new product. Therefore, the term “fermentation” includes conventional alcohol fermentation, which is typically performed using some type of living ferment, such as yeast, and involves the enzymatically controlled anaerobic conversion of simple sugars, including those produced through saccharification, into carbon dioxide and alcohol. Depending on the organic compounds employed and fermentative microorganism(s) employed, however, a host of other fermentation products may be generated in addition to, or in the alternative to, alcohol.
Recently, conversion of biomass through fermentation into ethanol or other useful products as a replacement for fossil fuels has garnered considerable attention. Biomass for such conversion processes can be potentially obtained from numerous different sources, including, for example, wood, paper, agricultural residues, food waste, herbaceous crops, and municipal and industrial solid wastes to name a few.
For a number of reasons, biomass is an attractive feedstock for producing fossil fuel substitutes. Biomass has a smaller carbon footprint than conventional fossil fuels because it typically comes from plants that have an annual growth cycle; therefore, the carbon dioxide liberated by the combustion of the derived fuel is subsequently reused through photosynthesis by the plant's regrowth and results in no net carbon dioxide in the earth's atmosphere. Further, biomass is readily available and the conversion of biomass provides an attractive way to dispose of many industrial and agricultural waste products. Finally, biomass is a renewable resource because crops may be grown on a continuous basis, utilizing the liberated carbon dioxide each cycle.
While biomass has the potential to provide an attractive fossil fuel alternative, substantial difficulties still remain. Because the main product of the fermentation is a commodity, namely fuel, production costs must be extremely low to be competitive with other fuels. In addition, a main goal of using biomass as a fossil fuel replacement is to reduce carbon pollution. Therefore, any conversion process used should require low energy input. Because the United States alone consumes approximately nine (9) million barrels of gasoline each day, the process of creating a usable fossil fuel replacement from biomass must be scalable to be a meaningful alternative.
Fermentation processes can be divided into two main categories, solid state fermentation (SSF) processes and submerged liquid fermentation (SLF) processes. Solid state fermentation processes involve growth of microorganisms on moist, solid biomass particles. The spaces between the particles contain a continuous gas phase and a non-saturated water phase. Thus, although droplets of water may be present between the particles in a solid state process, and there may be thin films of water at the particle surface, the inter-particle water phase is discontinuous and most of the inter-particle space is filled by the gas phase. The majority of water in the system, therefore, is absorbed within the moist solid particles. In submerged liquid processes by contrast, particles are disposed in a continuous liquid phase.
Although SSF has been practiced for hundreds of years in the preparation of traditional fermented foods, its application to the production of fermentation products within the context of modern biotechnology has been fairly limited. This is because historically it has been notoriously difficult to control the fermentation conditions within SSF. In practice, for example, temperature control, fluid channeling, excessive pressure drop, and evaporation have posed major problems to the development of a commercially viable SSF reactor and process that is suitable for large scale, industrial applications. Thus, while the process of SSF has been practiced at small, batch, scale in the Asian food and beverage industry for hundreds of years to make soy sauce and sake and research has been conducted more recently to produce other products such as enzymes, most fermentation processes used today are still carried out in SLF processes. Indeed, all commercial fermentation processes used for producing alternative fuels that exist today employ a SLF process.
Numerous drawbacks exist with using the SLF process, however. Two principal drawbacks of SLF processes is that they tend to be capital intensive and have high operating costs, making them less than optimum for producing many fermentation products, including alternative fuels, such as ethanol, on an industrial scale and at a competitive price.
If the foregoing problems associated with SSF could be resolved, or at least sufficiently ameliorated, a commercially viable SSF bioreactor and process that is suitable for large scale, industrial applications could be achieved. Such a SSF bioreactor and process could provide several advantages over existing SLF technologies, including high product yield, low cost, ease of use, and scalability.
A wide variety of apparatus have been tried as SSF bioreactors. These apparatus fall into two main categories: static systems and stirred systems. Stirred systems have a means for mixing the biomass during the fermentation process. Stirring adds complexity and significant cost to the bioreactor. This becomes especially true for a bioreactor device that is required to be scaled up to an industrial scale to support, for example, the fossil fuel alternative market.
Static systems are sometimes used because the microorganism used in the fermentation process can not withstand the disruption caused during stirring. Various static bioreactors for SSF have been designed and used including, flasks, petri dishes, columns and trays. These designs have been mostly for laboratory use and are not effective or efficiently designed to be scaled for use at an industrial level.
One of the major problems in utilizing a static SSF bioreactor on a large scale is temperature control. The fermentation of organic compounds in general, and sugars contained or released from biomass in particular, is an exothermic reaction, generating heat in the local area of the microorganism performing the conversion. This leads to localized elevated temperatures within the biomass in the reactor. The elevated temperatures within the SSF bioreactor can result in temperatures well above the optimum for microbial growth, which in turn can inhibit the fermentation process from occurring efficiently. Accordingly, a need exists for a SSF bioreactor design and method of using the same that permits temperature within the bioreactor to be maintained within acceptable process limits during the conversion process.
When a large volume of reacting biomass is confined to a conventional solid state reactor, large temperature gradients are established within the biomass volume. This is primarily due to the fact that it is difficult to remove the localized heat uniformly from the biomass using a remote heat sink. For example, if the walls of the bioreactor are a heat sink, a temperature differential will form radially from the center outward towards the walls. With scale-up, the conduction effect of the walls of the bioreactor will have little effect on the biomass in the center of the reactor and the radial temperature gradient will increase.
Temperature gradients also form in the axial direction. As the fermentation begins, heat from the exothermic reaction tends to rise. This creates a temperature gradient in the axial direction with the top of the biomass being hotter than the bottom.
In an attempt to control the temperature of the biomass, SSF bioreactors have been designed with forced aeration. The convection and evaporation effects of the gas as it passes through the biomass have been used to reduce the temperature. Air or gas is introduced at the bottom of the biomass in the SSF and flowed to the top. By controlling the temperature and humidity of the inlet gas, the biomass in the SSF can be cooled or heated respectively.
Numerous problems exist with present forced aeration bioreactor designs. First, the gas introduced at the bottom of the reactor tends to reduce the temperature of the biomass near the bottom of the reactor, but has a lesser effect on the biomass as it passes up through the reactor. As gas is introduced, it absorbs heat from the biomass at the bottom of the reactor, which in turn raises the temperature and humidity of the gas, and makes it less effective at cooling as it passes up through the reactor. This tends to bring the temperature of the biomass at the bottom of the reactor into equilibrium with the temperature of the input gas and creates an increasing temperature gradient as the height of the biomass increases. These effects are exacerbated as the height of the SSF increases. Furthermore, the pressure drop typically increases as the height increases making forced aeration more difficult.
Because of the problems with heat removal in forced aeration SSF bioreactors, the height of the bioreactor and therefore the height of the biomass has been kept low. It has been suggested that the height of the biomass in a forced aeration SSF bioreactor should not exceed one (1) meter. See D. A. Mitchell, et al., Solid State Fermentation Bioreactors, Fundamentals of Design and Operation, Chpt. 7, 93 (2006). This creates a problem, however, because by keeping the height small, large areas are required in order to scale up existing bioreactor designs, which in many cases will be impracticable due to the availability and/or cost of the required land.
One proposed solution to the height problem is suggested by Suryanarayan et. al. in U.S. Pat. No. 6,664,095 B1. The Suryanarayan patent suggest a tray stacking solution whereby the height of the biomass in each individual tray is kept small and a plurality of trays are stacked on top of each other. While this solution effectively keeps the height of the biomass small while allowing the bioreactor to increase in height, the tray stacking design and implementation is too expensive and impractical to scale to the industrial levels necessary for many potential applications, including for cost effective alternative fuel production.
A further problem with forced aeration SSF reactors is the drying effect of the aeration process. The water content of the biomass must be maintained. If the biomass becomes too dry, the efficiencies of the fermentation processes are reduced. Even if the gas entering the bioreactor is completely saturated, the biomass absorbs the moisture from the gas as it passes from the bottom of the bioreactor to the top and the resultant gas has a drying effect on the biomass. Further, the increase in temperature towards the top of the bioreactor can cause further evaporation, drying the biomass more.
In addition to the reduced efficiency of the fermentation processes, the drying of the biomass has a secondary effect. As the bed dries it will contract and reduce in volume. This reduction in volume will cause channeling and cause the biomass to pull away from the sides of the reactor. Channeling occurs when paths of lower resistance develop through the bed and the forced aeration flows through the bed along the channels only, rather than being evenly dispersed through the bed. Channeling can occur along the boundary between the reactor and the biomass or through the biomass itself. Channeling reduces the flow of gas to large parts of the volume of biomass causing localized temperature increases and an overall increase in the temperature gradients and thus, a reduction in process efficiency. As the bioreactor is scaled up, the bioreactor walls, which can be used as heat sinks, have less intimate contact with the biomass, increasing the temperature gradients in the radial direction.
Further, contemporary thinking is that liquid can not be effectively used in a static SSF bioreactor because the liquid can not be evenly dispersed throughout the biomass. The addition of liquid to static SSF reactors can result in flooding and inhibit the fermentation process. The permeability of biomass, depending on the source, is usually very limited and tends to decrease as the biomass depth is increased. Further, as the biomass is fermented, the biomass degrades, its volume decreases, and its density increases, further reducing permeability and inhibiting fluid flow.
Stirring or otherwise mixing the biomass in the bioreactor can reduce channeling, help eliminate temperature gradients, allow liquid to be added to the biomass, and more evenly distribute the moisture in the reactor. While stirring can have positive effects, stirring mechanisms are complicated to build and become extremely expensive to construct and operate when scaled. Even if stirring equipment on a large scale is effectively designed, the process of stirring will be extremely expensive for a large scale SSF reactor. Wet biomass requires large amounts of energy to mix or stir because of its weight. In addition, as mentioned above, stirring can have a deleterious effect on the microorganisms used in the fermentation process.
In view of the foregoing, a need exists for an improved static solid state bioreactor that addresses or at least ameliorates one or more of the problems associated with existing SSF bioreactor designs.
Saccharification is the process of breaking down a complex carbohydrate (such as starch, cellulose or hemicellulose) into its monosaccharide components or sugars. Saccharification can be facilitated via the use of chemical reagents, biological agents, or combinations of these two. During alternative fuel production processes, the converted biomass is typically subjected to a saccharification process prior to or simultaneous with the fermentation process used to convert the simple sugars in the biomass, including those released through saccharification, into carbon dioxide and alcohol and/or methane. Accordingly, because one of the major potential applications of an industrial scale static SSF bioreactor is the production of alternative fuels, such as ethanol and/or methane, it would be beneficial if such a bioreactor could also be used for saccharification of biomass, either separate from the fermentation process or simultaneous with the fermentation process.